Thermoplastic elastomers are known to exhibit rubber elasticity at room temperature and can readily be plasticized, melted, and thus molded upon heating. Also, thermoplastic elastomers can be recycled. For these reasons, thermoplastic elastomers have recently become widely used in various fields, including automobile parts, home electric appliances, toys, sports equipment, and articles of daily use.
Among other thermoplastic elastomers, styrene thermoplastic elastomers, such as styrene-butadiene-styrene (SBS) block copolymers, styrene-isoprene-styrene (SIS) block copolymers and hydrogenated products thereof, are particularly widely used since these elastomers are less expensive and have a better flexibility. Also widely used are thermoplastic polyurethane elastomers (TPU), which have a high abrasion resistance and a better mechanical strength.
Many proposals have been made thus far to improve physical properties of the styrene thermoplastic elastomers. For example, an improved thermoplastic elastomer composition is known that contains a styrene thermoplastic elastomer, an ethylene-α-olefin copolymer and a hydrocarbon oil (e.g., Japanese Patent Laid-Open Publications No. Hei 8-231817, Hei 9-316287, Hei 10-53688 and Hei 11-21415). Although the thermoplastic elastomer compositions described in these publications have a high moldability and molded products made from these compositions exhibit a high softness, rubber elasticity, mechanical strength, and, in some cases, a high transparency, none has shown sufficient scratch resistance and/or abrasion resistance. Some of these compositions even lack desired oil resistance. For these reasons, the thermoplastic elastomer compositions are not necessarily suited for applications where high scratch resistance, abrasion resistance and oil resistance are required, in particular for the application of footgear soles. Such a drawback needs to be addressed.
Another type of thermoplastic elastomer composition is known, which is mainly used for the purpose of the slush molding and is composed of a styrene thermoplastic elastomer, two different types of ethylene-α-olefin copolymers and a polypropylene resin composition (Japanese Patent Laid-Open Publication No. Hei 10-279738). This thermoplastic elastomer composition, however, lacks sufficient softness and rubber elasticity, and thus is not necessarily suited for applications where a particular property of elastomer (elasticity) is of significant importance.
There is also known a polymer composition that is prepared by adding an organic peroxide to a particular thermoplastic elastomer composition and then melting and kneading the mixture. This thermoplastic elastomer composition contains a styrene thermoplastic elastomer, a rubber softening agent, a polyethylene or an ethylene copolymer, which has been polymerized by a single-site catalyst, and a propylene polymer (Japanese Patent Laid-Open Publications No. Hei 10-251480 and 2000-17141). However, the polymer composition described in these publications also lacks sufficient softness or rubber elasticity and thus is not sufficiently suited for applications where a particular property of elastomer (elasticity) is of significant importance. The polymer composition also needs to be improved since the molded products formed from the polymer composition have a slimy texture and the polymer composition lacks sufficient mechanical strength.
As for the shoe soles made of the thermoplastic elastomer, various proposals have been made thus far in an effort to achieve physical properties suitable for shoe soles. For example, a thermoplastic resin composition for use in shoe soles has been developed with the intention of improving weather resistance, heat resistance and oil resistance. This thermoplastic resin composition is prepared by first obtaining a composition by adding to a particular styrene thermoplastic elastomer, a rubber softening agent, a particular polypropylene polymer and, if necessary, polyethylene, and then crosslinking the composition by an organic peroxide (Japanese Patent Laid-Open Publication No. 2000-139503). Another type of thermoplastic elastomer composition intended for use in shoe soles or other applications contains an epoxidized diene block copolymer and a polyurethane elastomer (Japanese Patent Laid-Open Publication No. 2000-186197). Also known is a shoe sole made of a block copolymer consisting of polymer blocks of isobutylene and polymer blocks of a vinyl aromatic compound (Japanese Patent Laid-Open Publication No. 2000-290331).
However, none of the above-described known compositions for use in shoe soles or none of the shoe soles made of such a composition is known to simultaneously possess all of the essential performances required for footgear soles, such as shoe soles, in particular such performances as high abrasion resistance, hydrolysis resistance, grip, shock absorbance, oil resistance, mechanical strength, softness and lightweightness. Thus, these compositions are far from satisfactory for use in shoe soles.
Accordingly, it is an objective of the present invention to provide a thermoplastic elastomer composition that simultaneously possesses high moldability, abrasion resistance, scratch resistance, hydrolysis resistance, softness, rubber elasticity, grip, shock absorbance, oil resistance and lightweightness, and can thus be effectively used in various applications.
It is another objective of the present invention to provide a thermoplastic elastomer composition for use in footgear soles, as well as a footgear sole, that simultaneously possesses all of the essential properties required for footgear soles, namely, high abrasion resistance, scratch resistance, hydrolysis resistance, grip, shock absorbance, oil resistance, mechanical strength, softness and lightweightness.